Synthesis, Stability, and (De)hydrogenation Catalysis by Normal and Abnormal Alkene- And Picolyl-Tethered NHC Ruthenium Complexes

Article abstract of DOI:10.1021/acs.organomet.9b00178

A series of p-cymene and cyclopentadienyl Ru(II)-aNHC complexes were synthesized from 2-methylimidazolium salts with either an N-bound alkenyl (1, 3) or picolyl tether (6, 7). The C(5)-Me substituted alkenyl-tethered analogues (2, 4) were also synthesized. Ag-mediated C(2)-dealkylation was a prominent side reaction that led to the formation of normally bound NHC Ru(II) complexes, which in selected cases were isolated (5, 8). A C(4)- over C(2)-selectivity for ruthenium binding was established by protecting the C(2)-position with an iPr group on the imidazolium precursor, for which unique p-cymene (9) and cyclopentadienyl (10) Ru(II)-aNHC derivatives were synthesized. All complexes were applied in the transfer hydrogenation of ketones and in secondary alcohol oxidation, with higher catalytic activity for the p-cymene over the cyclopentadienyl systems, as well as the alkenyl- over the picolyl-containing aNHC complexes.

Full text of DOI:10.1021/acs.organomet.9b00178

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis, Stability, and (De)hydrogenation Catalysis by Normal and
Abnormal Alkene- and Picolyl-Tethered NHC Ruthenium Complexes
Frederick P. Malan,† Eric Singleton,† Petrus H. van Rooyen,† Martin Albrecht,*,‡
,†
́
and Marile Landman*
†Department of Chemistry, University of Pretoria, 02 Lynnwood Road, Hatfield, Pretoria 0002, South Africa
‡
̈
Department of Chemistry and Biochemistry, Universitat Bern, Freiestrasse 3, 3012 Bern, Switzerland
S
* Supporting Information
ABSTRACT: A series of p-cymene and cyclopentadienyl Ru(II)-aNHC
complexes were synthesized from 2-methylimidazolium salts with either an N-
bound alkenyl (1, 3) or picolyl tether (6, 7). The C(5)-Me substituted
alkenyl-tethered analogues (2, 4) were also synthesized. Ag-mediated C(2)-
dealkylation was a prominent side reaction that led to the formation of
normally bound NHC Ru(II) complexes, which in selected cases were
isolated (5, 8). A C(4)- over C(2)-selectivity for ruthenium binding was
established by protecting the C(2)-position with an iPr group on the
imidazolium precursor, for which unique p-cymene (9) and cyclopentadienyl
(10) Ru(II)-aNHC derivatives were synthesized. All complexes were applied
in the transfer hydrogenation of ketones and in secondary alcohol oxidation, with higher catalytic activity for the p-cymene over
the cyclopentadienyl systems, as well as the alkenyl- over the picolyl-containing aNHC complexes.
INTRODUCTION
■
N-Heterocyclic carbenes (NHCs) have the ability to exhibit
both innocent and noninnocent behavior in metal-mediated
transformation reactions.1−3 Therefore, it is not surprising that
these stabilizing ligands have been coordinated to most of the
transition metals in a range of different oxidation states. NHCs
are known to form some of the strongest M−C bonds,4 5 and
,
hence attempts have been directed toward the synthesis of
highly electron-donating NHCs to better stabilize sensitive
transition-metal species, as well as to access high oxidation
state metal species.2,3 It has also been shown that C(4)/C(5)-
bound imidazolylidenes (a subclass of so-called abnormal
NHCs, aNHCs) are significantly stronger donors compared to
their classical C(2)-bound imidazolylidene counterparts
(Figure 1).3 The enhanced donor properties imparted by
aNHCs has been shown to further improve the catalytic
efficiency of these metal complexes in a variety of reactions.2,4
A range of different backbone- and N-functionalized aNHCs
have been reported with tunable donor properties that are
significantly influenced by both the number and positions of
heteroatoms within the heterocycle (Figure 1), with
subsequent catalytic implications.5 For example, the pro-
nounced mesoionic character of the aNHC ligand allows for
the usual carbene-type stabilization when coordinated to low-
valent electron-rich metals, whereas a carbanionic character
may be dominant with electron-poor, high oxidation state
metal centers.2,6
Figure 1. Order of donor strength of imidazolylidene and
pyrazolylidene-based NHCs.
acidity of the imidazolium C(4)/C(5) proton,7 which requires
specific protocols to direct metalation at this position.7,8
Protection of the imidazolium C(2) position with alkyl or
aryl substituents is probably the most rational and useful route
that has been developed for selective metal coordination to the
imidazolylidene C(4) atom.4,5a However, the formation of
C(4)-bound aNHCs via transmetalation of the corresponding
Ag-aNHC intermediate is generally limited because of redox
reactions of the imidazolium salt with the strong oxidant
Ag2O.3 The need for more facile routes to access these
Despite these attractive advantages of aNHCs, the organo-
metallic chemistry of NHCs remains dominated by normally
bound NHC complexes.4 A major limitation of aNHC
complexes is the synthetic accessibility because of the low
Received: March 15, 2019
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of the aNHC Ligand Precursors
desirable aNHCs remains relevant, as the rational preparation
of aNHC metal complexes continues to be a synthetic
challenge.4,9,10 One attractive strategy, particularly for
aNHCs, is chelate-assisted C−H bond activation.11 In this
approach a variety of donor groups such as alkenyl, picolyl,
pyridyl, thioether, amine, oxo, or phosphine substituents are
grafted onto the C(2)-protected NHC scaffold, usually
through N-alkylation, essentially forming a bidentate aNHC
ligand precursor. Abnormal coordination selectivity has also
been related to steric control imparted by the tether length and
the bite angle, as well as to the nature of the anion of the
aNHC precursor.12 In addition, such N-substituted chelators
may exhibit hemilability and noninnocent behavior in solution,
which enhances the set of metal-specific steps within a
potential catalytic cycle.13 In light of the latter, the lack of
exploitation of potentially hemilabile bidentate aNHC ligand
systems prompted us to evaluate the catalytic potential of
chelating aNHC ligands when bound to a ruthenium metal
center. Even though only a small number of well-defined
aNHC Ru complexes have been reported, most of these
complexes exhibit superior catalytic activities compared to
their normally bound analogues.3a,14 Here we report the
synthesis of eight new abnormally bound NHC half-sandwich
Ru(II) complexes and demonstrate the strong binding of these
aNHC ligands through acid stability studies. In addition, the
novel complexes were evaluated for their application in transfer
(de)hydrogenation catalysis. Each of these Ru(II) complexes is
comprised of either an alkene- or picolyl-substituent as one of
two possible N-functionalized chelating moieties. In addition,
selectivity and yield issues typically encountered with Ag2O-
assisted carbene transfer reactions are addressed.
tion reaction to produce the C(2)-protected imidazolium
chloride salts in excellent yields (93%, [HL1]Cl; 88%,
[HL2]Cl). The tetra-substituted imidazolium salt [HL3]Cl
was accessed by reacting 2,4-dimethylimidazole first with acetic
anhydride, followed by methyl iodide, to selectively yield the
1,2,5-trimethyl imidazole. It was found previously that direct
deprotonation and alkylation of 2,4-dimethylimidazole gave
mixtures of the 1,2,4-, and 1,2,5-trisubstituted imidazoles.15
The 1,2,5-trimethylimidazole precursor was further reacted
with 3-chloro-2-methyl-propene to obtain [HL3]Cl in
moderate yield (53%). In an attempt to address selectivity
issues in the metalation (see below), the symmetrical aNHC
precursor [HL4]Cl was synthesized in high yield by sequential
treatment of 2-isopropylimidazole with KOH and 3-chloro-2-
methylpropene, followed by another equivalent of 3-chloro-2-
methyl-propene (90%).
1
The H NMR spectra of [HL1]Cl, [HL3]Cl, and [HL4]Cl
all showed characteristic singlets for the 2-methylpropenyl
moiety at δH 1.53−1.78 (CH3), 4.60−4.82 (NCH2), and
4.51−5.06 (two singlets, CH2). Depending on the sym-
metrical nature of the ligand, the imidazolium backbone
protons either appeared as a singlet, at δH 7.47 ([HL4]Cl) or
7.14 ([HL3]Cl), or as two singlets (δH 7.51 and 7.78 for
[HL1]Cl, 7.46 and 7.64 for [HL2]Cl).16 Although only 13C,
15N, 31P, and 77Se NMR techniques are classically used to
compare σ-donor and π-accepting properties of various NHC
ligands,17 it is interesting to compare the values of the aromatic
1
imidazolium H NMR signals of L1−L4 to gauge σ-donor
strength: L1 < L2 < L4 < L3. According to this series, ligand
L3 bearing a C(5)-Me group is more donating than its C(5)-H
counterpart, L1, as might be expected from the inductive effect
of a methyl substituent. This approximate donor strength series
is supported when comparing the increasing resonance
frequency of the precarbenic C(4)-signal in the 13C NMR
spectra in the series 122.4 ppm ([HL2]Cl), 122.9 ppm
([HL1]Cl), 123.7 ppm ([HL4]Cl), 129.7 ppm ([HL3]Cl).
Synthesis of the Normal and Abnormal NHC Ru(II)
Complexes. Each of the ligands L1−L4 contains a C(2)-alkyl
RESULTS AND DISCUSSION
■
Synthesis of the NHC Ligands. The four aNHC ligand
precursors [HL1]Cl−[HL4]Cl were synthesized from the
respective C(2)-substituted imidazoles (Scheme 1). For
example, 1,2-dimethylimidazole reacts with either 3-chloro-2-
methyl-propene or 2-(chloromethyl)pyridine in a quaterniza-
B
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Scheme 2. Synthesis of aNHC Half-Sandwich Ru(II) Complexes with an Alkenyl Chelating Group
Scheme 3. Synthesis of Picolyl-Functionalized aNHC Half-Sandwich Ru(II) Complexes
substituent to inhibit metal coordination at the normal
position of the NHC. Metalation was performed using
standard literature procedures,3,18 which involves reacting the
imidazolium salt with Ag2O under exclusion of light to form
the carbene silver complex in situ, and subsequently treating
the reaction mixture with the appropriate ruthenium(II)
precursor (Scheme 2). This procedure afforded the aNHC
ruthenium(II) complexes 1 and 2 from the reaction of [HL1]
Cl and [HL3]Cl, respectively, with [(p-cymene)RuCl2]2 in
moderate yield (49% and 38%, respectively).19 Trans-
metalation with [CpRuCl(PPh3)2] gave the aNHC complexes
3 and 4, respectively. While complex 3 was the only detectable
product when starting from the trisubstituted imidazolium salt
[HL1]Cl, the tetrasubstituted imidazolium salt [HL3]Cl
afforded, in addition to the aNHC complex 4, also the normal
NHC complex 5. Similar C(2)−Calkyl bond cleavage was
observed previously,3b in particular, with methyl, ethyl, and
benzyl substituents at C(2).4b Conversely, no dealkylation
occurs when the C(2) substituent is a bulky, secondary alkyl or
an aryl group such as isopropyl or phenyl.1d,3b The yields of
complexes 1−5 were only moderate (19−49%), and complexes
1 and 2 formed slightly better (>38% yields) owing to the
higher reactivity of [(p-cymene)RuCl2]2 versus [CpRuCl-
(PPh3)2].
spectator p-cymene or cyclopentadienyl (Cp) ligand appeared
in an equimolar ratio to the carbene ligand. In addition, a slight
upfield shift of the C(5)−H proton was observed from δH 7.51
in [HL1]Cl to 7.41 (1) and 7.02 (3), respectively, indicative of
higher electron density on the C(4) position. Alkene
1
coordination was also confirmed by H NMR spectroscopy,
as the signals of the symmetric NCH2 methylene protons from
the ligand salts (singlet around δH 4.7) split into AB doublets
upon coordination (2JHH = 9−15 Hz) due to their
diastereotopic nature. The generally large chemical shift
differences between the coupling doublets (Δδ up to 1.6
ppm) indicates distinctly different chemical environments for
the two NCH2 protons, which suggests rigid alkene
coordination.
Despite successive attempts, complete separation of 4 and 5
by either column chromatography or solvent extraction has
failed so far. The 1H and 31P NMR spectrum of the mixture of
abnormal/normal NHC complexes indicates an ∼2:1 ratio of 4
and 5. Characteristically, the aNHC complex 4 features an
NMR signal at δH 2.92 for the C(2)−CH3 protons, whereas
complex 5 shows a diagnostic resonance at δH 6.74 for the
C(4)-bound proton. The presence of a methyl substituent on
C5 in ligand L3 leads to a substantial preference for C(2)-
dealkylation and provides the normally bound NHC as the
major product. This result is attributed to a lower acidity of the
C(4)−H proton in [HL3]Cl (cf. NMR data), which reduces
the deprotonation propensity of Ag2O in favor of C(2)−Me
oxidation.
Spectroscopic evidence of aNHC coordination in complexes
1
1−4 was obtained from the disappearance of the low-field H
NMR signal due to the C(4)-H proton and the downfield shift
of the corresponding 13C NMR resonance from ∼125 ppm to
1
147.0−171.1 ppm. Moreover, the H NMR signals for the
C
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When the type of chelate was changed from an olefin to
pyridine, that is, employing [HL2]Cl with an N-picolyl
substituent, provided a similar outcome and afforded
complexes 6−8 in moderate yields (Scheme 3). Even though
imine interactions with Ag+ are more prominent,12 yields of
aNHC-Ru(II) complexes did not appreciably increase
compared to complexes 1−4 with an olefin wingtip
functionality. Again, abnormal NHC coordination was
1
indicated by the disappearance of the H NMR signal of
C(4)−H (δH 7.64 in [HL2]Cl), along with a slight upfield shift
of C(5)−H (δH 6.58 and 6.64 for 6 and 7, respectively), as well
as the deshielding of the corresponding C(4)−Ru carbene
signal to δC 157.8 and 159.9, respectively. Chelation of the
picolyl moiety was supported by the characteristic downfield
shift of the ortho proton of the pyridyl substituent (δH 9.36 for
6; 8.25 for 7) and by the splitting of the methylene linker into
an AB signal (2JHH = 15−18 Hz). Separation attempts of
complexes 7 and 8 failed, even after numerous attempts at
recrystallization and column chromatography. Distinct NMR
signals of the C(4/5)−H at δH 6.34 and 6.44 as well as a low-
field resonance at δC 184.4 for the C(2)−Ru nucleus indicate
the presence of the normally bound NHC ruthenium complex
8 in approximately equal quantities (ratio 7/8 is 0.46:0.54
1
according to H and 31P NMR integration).
Figure 3. ORTEP plots of complexes 5−8. Thermal ellipsoids are
drawn at 50% level. For clarity, the phenyl moieties of PPh3 are shown
as wireframe presentations, and noncoordinating anions and hydro-
gens are omitted (structures of 7 and 8 were obtained from a
cocrystal).
Crystallographic Characterization of the Complexes.
The molecular structures and carbene bonding modes deduced
from NMR studies were unequivocally supported by X-ray
diffraction studies for complexes 1, 3, and 5−8 (Figure 2,
the aNHC complexes (average Ru−C 2.05(1) Å for complexes
1, 3, 6, 7) is shorter than the Ru−C(2) bond distance in the
normal NHC complex 8 (Ru−C 2.099(4) Å). This might
either point to a stronger Ru−C(4) bond (abnormal vs normal
NHC) or may be due to the presence of a potentially shielding
pseudo-ortho methyl group in 8. All Ru−Ccarbene bond
distances are in good correlation with the bond distances of
other related normal18 and abnormal21,23 NHC Ru(II)
complexes.
The difficulty in separating complexes 7 and 8 is
demonstrated by the fact that crystallization led repetitively
to cocrystallization and afforded crystals that contained both
complexes 7 and 8. Two different unit cells were obtained from
X-ray diffraction analysis, each with unique disorders, when
either CH2Cl2 or CHCl3 was used as crystallization solvent
(Figures S36 and S37). These crystals constitute one of the
rare examples where, regardless of the solvent used, disordered
crystals contain two cocrystallized complexes as unique
polymorphs. Co-crystallization may be facilitated by the similar
steric environment that the methylimidazole and pyridyl
moieties of the NHC ligand exhibit,24 which renders these
two entities of the chelating ligand almost identical in each of
the complexes.
Selectivity Aspects of C4/C5−H Activation. Different
effects may contribute to the low yields obtained for the
abnormal carbene complexes, including (i) occurrence of side
reactions due to the relatively strong C4−H bond, (ii) the
strongly oxidizing nature of Ag2O that may cause ligand
degradation and/or C2-metalation, and (iii) lack of selectivity
between C4/C5-unsubstituted ligand precursors, and ensuing
divergence of product stability as a result of chelation versus
monodentate bonding. We reasoned that the symmetric
carbene precursor ([HL4]Cl), that is, the ligand bearing
stabilizing alkenyl units adjacent to both the C(4) and the
Figure 2. ORTEP plots of compounds 1 and 3. Thermal ellipsoids are
drawn at 50% level. For clarity, the phenyl moieties of PPh3 are shown
as wireframe presentations, and noncoordinating anions and hydro-
gens are omitted.
Figure 3).20,21,22 All of the half-sandwich Ru(II) complexes
assumed the typical three-legged piano stool structures.18
Selected bond lengths and angles are summarized in Table 1
and Table S4 (Supporting Information). If chelation of the
alkene tether in complexes 1−3 is considered to form
pseudofive-membered ruthenacyles, the resulting bite angles
vary between 89.49(1) and 90.45(2)°, marginally wider than
with the normally bound NHC in 5 at 89.10(1)°. The picolyl-
containing complexes 6−8 feature a six-membered metalla-
cycle and exhibit slightly more acute chelate bite angles of
85.7(3)−86.6(2)°. They exhibit synclinal torsion angles
between the pyridyl and imidazolylidene mean planes between
−48.60(3) and −55.08(6)°, while the alkene-containing
complexes (1, 3, 5) exhibit anticlinal torsion angles in the
−108.369(4)° to −122.675(2)° range. This inclination renders
the NHC and the alkene units closer to a coplanar
arrangement. Interestingly, the Ru−C(4) bond distance of
D
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Table 1. Selected Bond Lengths (Å) and Angles (deg) of Complexes 1, 3, 5−8, and 10
1
3
5
6
7
8
10
a
a
Ru1−C4
Ru1−Cg
Ru−Cl1
C4−Ru1−E
2.045(3)
1.728(3)
2.4114(7)
2.0586(2)
1.866(3)
2.032(4)
1.905(3)
2.045(2)
1.691(2)
2.4075(6)
2.057(6)
1.853(6)
2.099(4)
1.856(6)
2.046(2)
1.899(4)
b
c
c
ca
,
d
d
a d
,
c
89.49(1)
90.45(2)
89.10(1)
85.94(8)
86.6(2)
85.7(3)
91.31(8)
a
b
c
C2 replaces C4 for C(2)-bound (normal) NHC. Cg = centroid of arene/cyclopentadienyl moiety. E = Average position between two alkenyl
d
carbon atoms. E = Nitrogen atom of pyridyl moiety.
Scheme 4. Formation of the C(2)-Isopropyl Functionalized Ru(II)-aNHC Complexes 9 and 10
C(5) position, alleviates the drawbacks from the low selectivity
of Ag carbene formation. Furthermore, L4 features a C(2)-
isopropyl substituent that is resistant to Ag-mediated deal-
kylation,3b which further minimizes formation of side products
such as normal carbene complexes. Reaction of [HL4]Cl with
Ag2O and [(p-cymene)RuCl2]2 or [CpRuCl(PPh3)2] gave
complexes 9 and 10, respectively (Scheme 4). Yields were
indeed significantly higher (62% and 55% for 9 and 10,
respectively) and without formation of detectable side
products.
Similar to the aNHC ruthenium complexes described above,
complexes 9 and 10 showed distinct signals of abnormal
carbene bonding in their NMR spectra, including the
disappearance of the C(4)−H signal at δH = 7.47, an upfield
shift of the C(5)−H resonance to δH = 7.17 (9) and 6.75 (10),
and a downfield shift of the C(4)−Ru signal to δC = 152.3 (9)
and 148.6 (10). Chelation was deduced from the shift of the
olefinic signals from δH = 4.58 and 5.06 ([HL4]Cl, Δδ = 0.48
ppm) to 4.03 and 5.51 (9, Δδ = 1.48 ppm), and to 2.27 and
3.65 (10, Δδ = 1.38 ppm). The molecular structure of 10
further confirmed the bidentate nature of L4, with a
pseudobite angle of 91.31(8)° and a Ru−C(4) bond distance
of 2.046(2) Å (Figure 4), similar to the analogous complexes 1
and 3.
Acid Stability. To probe the robustness of the Ru−C(4)
bond in the novel aNHC complexes, the stability of these
complexes in an acidic environment was investigated for
complexes 1 and 3 as representatives of p-cymene and
cyclopentadienyl systems, respectively. Figure 5 shows the
1H NMR spectra of 1 in acetone-d6 upon addition of various
equivalents of DCl (1 M, D2O solution). While up to 12 equiv
of DCl did not indicate any cleavage of the Ru−C4 bond, three
Figure 4. ORTEP plot of compound 10. Thermal ellipsoids are drawn
at 50% level. For clarity, the phenyl moieties of PPh3 are shown as
wireframe presentations, and noncoordinating anions and hydrogens
are omitted.
distinct events were identified upon DCl titration of a solution
of complex 1:
(i) Rapid H/D isotope exchange at C5: Addition of 1 equiv of
DCl led to an immediate decrease of the signal at δH
7.41 only and to full disappearance after addition of 6
equiv DCl, indicating deuteration of the C(5) position.
This reactivity was observed in similar aNHC complexes
of Ir and Rh, and it is indicative of the noninnocent
character of the aNHC ligand.15 For example, it has
been suggested that the strongly mesoionic character of
the imidazolylidene ligand may facilitate dearomatiza-
tion. This is followed by the addition of a proton to a
formal metalla-allyl anionic fragment.15a The deuteration
at C(5) is attributed to the presence of DCl, since a
E
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1
▲
Figure 5. H NMR spectra of complex 1 upon titration with DCl showing resonances of complex 1 (a−d), free p-cymene ( ), and an aNHC Ru
●
complex with noncoordinating N-alkenyl tether ( ).
5.31, all with identical coupling constant 3JHH = 6 Hz, in
an approximate ratio of 1:4:4. Of the remaining seven
singlets (δH 5.07, 4.90, 4.77, 3.98, 2.75, 2.11, and 1.27),
the four signals at δH 5.07, 4.90, 4.77, and 1.27
correspond to the free alkenyl tether, suggesting
monodentate coordination of the aNHC ligand to the
Ru(II) center. Such bonding indicates that all organic
ligands in complex 1 are more labile in an acidic
environment than the aNHC ligand. After addition of
more than 12 equiv of DCl and over a period of 10 d,
the 1H NMR spectrum reveals signals that suggest that a
remarkably acid- and moisture-stable aNHC Ru(II)
complex remains in solution, although it has lost the p-
cymene ligand, and the N-alkenyl tether is unbound. A
robust Ru−CaNHC bond is further supported by the
carbenic resonance at δC 171.1, which remained present
over the whole duration of this experiment. Even though
the formed compound appears to be stable, attempts to
isolate it have not been successful.
control experiment using only D2O (>12 equiv) resulted
in no deuteration of the ligand at all. The protio versus
deuterio ratio at C(5) follows a rapid decrease after
addition of DCl from 80% (1 equiv) to 29% (2 equiv),
13% (4 equiv), and <1% (8 equiv). No formation of the
free aNHC ligand or the corresponding imidazolium salt
was observed, even upon heating of the solution to 40
°C for several hours, which underpins the robustness of
the Ru-carbene bond.
(ii) p-Cymene dissociation: Already in the presence of 1 equiv
3
of DCl, new signals appeared at δH 7.09 (q, JHH = 6
3
Hz), 2.84 (m), 2.25 (s), and 1.20 (d, JHH = 9 Hz),
corresponding to free p-cymene (17%; full spectra in
Figures S3 and S4). In agreement with cymene loss, the
intensity of the doublets corresponding to the
coordinated p-cymene ligand gradually decreased (cf.
δH 6.65, 6.50, 5.80, 5.27). The relative amount of bound
p-cymene was 83% upon addition of 1 equiv of DCl (2
h) and 70% in the presence of 8 equiv of DCl (1 d). The
signals for the coordinated p-cymene ligand do not fully
disappear even after addition of 12 equiv of DCl, which
might be indicative of an equilibrium existing between
free and bound p-cymene. Deuteration of the C(5)−H is
not expected to affect p-cymene dissociation, especially
since the signals of coordinated p-cymene remain after
full deuteration of C(5).
A similar H/D isotope exchange was observed with the
analogous cyclopentadienyl-containing complex 3, where an
immediate deuteration of the C(5)−H proton was indicated
by the loss of the signal at δH 7.02. No sign of deuteration of
the cyclopentadienyl or the alkenyl unit was noted. Instead,
complex 3 was fairly stable in an acidic environment with the
cyclopentadienyl, carbene, and N-alkenyl tether remaining
intact upon increasing the amounts of added DCl up to 12
equiv. Addition of DCl also led to the appearance of new,
unassigned signals at δH 5.27, 5.35, and 5.62. However, no
support for the loss of cyclopentadiene was obtained even over
prolonged periods of heating to 40 °C, and only a gradual
deuteration of all signals was observed after two weeks.
(iii) Alkene dissociation: Dissociation of the p-cymene ligand
generates three available coordination sites on the
ruthenium(II) center. With increasing quantities of
aqueous DCl, the propensity for the formation of
(aNHC)Ru−OD2 species increases.25 After addition of
more than 8 equiv of DCl, a series of 10 new signals
appear including three doublets at δH 6.01, 5.46, and
F
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a
Table 2. Transfer Hydrogenation of Benzophenone using Complexes 1−10
b
conversion (%)
c
entry
complex
NHC N-tether
spectator ligand system
2 h
6 h
TOFini (h−1
)
1
2
3
4
5
6
7
8
9
1
2
3
4/5
6
7/8
9
10
alkenyl
alkenyl
alkenyl
alkenyl
picolyl
picolyl
alkenyl
alkenyl
p-cym/Cl
p-cym/Cl
Cp/PPh3
Cp/PPh3
p-cym/Cl
Cp/PPh3
p-cym/Cl
Cp/PPh3
p-cym/Cl
Cp/PPh3
63
52
32
37
52
30
60
42
nd
nd
93
76
69
50
73
50
89
61
60
38
26
24
34
22
58
46
d
d
e
e
f
e
[(p-cym)RuCl2]2
[CpRuCl(PPh3)2]
8
f
nd
nd
e
10
6
a
General conditions: Benzophenone (0.6 mmol), iPrOH (5 mL), base (5 mol %), [Ru] (1 mol %), reflux. Base used depending on Ru catalyst:
b
c
KOH (1, 2, 6, 9); KOtBu (3−5, 7, 8, 10). Determined by GC, based on the average of at least two runs. Determined after the first 15 min of
reaction time. Mixture of complexes as isolated after purification. nd = not detected. After 18 h.
d
e
f
Transfer Hydrogenation. Similar to the series of normal-
and abnormal-NHC Ru(II) complexes described in the
literature,26 complexes 1−10 were active in the transfer
hydrogenation of selected ketone compounds. Standard
conditions were employed, that is, excess isopropyl alcohol
as a hydrogen donor, base (KOH/KOtBu) as activator, and
conditions). The efficiency of the transfer hydrogenation
reaction is temperature- and base-dependent. Under base-free
conditions, only low conversions were obtained that did not
exceed 30% (18 h). Similarly, less than 10% benzophenone was
converted after 6 h, when the reaction was conducted at room
temperature. A blank reaction in the absence of a catalyst
showed negligible background conversion with 4% product
observed after 18 h (5 mol % KOtBu). A change of the base
from KOH to KOtBu induced a marginal increase of
conversion for the cymene-containing complexes (6% after 2
h). The effect is much more substantial with the cyclo-
pentadienyl Ru complexes, which are only very poorly active in
the presence of KOH and required KOtBu to induce catalytic
turnover.
Introduction of a methyl group at C(5) of the aNHC ligand
(complex 2) lowered the performance slightly when compared
to the analogue containing a C(5)−H unit (complex 1, cf.
entries 1 and 2), despite the fact that the aNHC ligand in 2 is
slightly more donating. The lower performance may therefore
be attributed to steric effects, which are particularly relevant for
inner-sphere mechanisms. Changing the chelate from an N-
alkene tether to an N-picolyl substituent (complex 6) reduces
the catalytic activity of the ruthenium center (entry 1 vs 5),
which highlights the importance of chelate group tailoring. The
C(2)−iPr functionalized NHC Ru complex 9 performed
essentially identical to complex 1, indicating that the C(2)-
substituent (Me vs iPr) on the aNHC ligand does not affect
the catalytic activity of the metal center (entry 7). The catalytic
activity of the abnormal/normal NHC Ru mixtures (4/5 and
7/8) was consistently lower after 6 h, compared to that of the
purely aNHC Ru complexes with a Cp spectator ligand (3, 10;
cf. entries 3, 4, 6, and 8), hinting to a beneficial effect of the
abnormal NHC bonding mode.
1
benzophenone as model substrate. Both H NMR and gas
chromatography (GC) analyses of reaction mixtures in the
presence of suitable internal standards suggested a selective
reaction, and hence substrate conversions correspond to
product yields. After reaction optimization involving catalyst
loading, base, and reaction time (see Table S5), complexes 1−
10 were screened for their catalytic activity. With complexes 1
(p-cymene) and 3 (cyclopentadienyl) as catalysts, initial
turnover frequencies TOFini = 60 and 26 h−1, respectively,
were determined (TOFini after 15 min reaction time; Table 2,
entries 1 and 3). While these rates are ∼3 orders of magnitude
lower than some of the best-performing ruthenium-based
catalysts,2c,26 they are comparable to similar bidentate NHC
Ru(II) analogues.2c,11b Nonetheless, complexes 1 and 3
performed considerably better than the precursor salts (entries
9 and 10), revealing a direct impact of the tethered aNHC
ligand on the catalytic activity.
Although chelation generally renders a more robust and
longer-lived catalyst, it might inhibit substrate binding and
hence reduce the overall yield.2c,26 Both complexes 1 and 3
demonstrate good longevity and provide high yields after 6 h
(93% for complex 1) and continued activity of complex 3,
which reaches a mere 50% conversion after 6 h but remains
active and accomplishes 81% conversion after 18 h. When
classes of complexes are compared, the p-cymene Ru(II)
systems consistently outperformed the cyclopentadienyl Ru-
(II) analogues with the same NHC ligand (compare entries 1
vs 3; 2 vs 4; 5 vs 6; 7 vs 8), presumably because alkoxide
coordination is easier by substitution of the chloride ligand
than by replacing any of the ligands in the Cp complexes. For
the cyclopentadienyl Ru(II) complexes to be catalytically
active, the most obvious activation pathways include either
alkene or phosphine dissociation, which is disfavored due to
chelation and the strong Ru−P bond, respectively (cf. the
stability of the alkenyl tether in the Cp complexes under acidic
Variation of the substrate demonstrates that steric and
electrochemical differences in the substrate notably affect
conversion (Table 3); that is, less bulky substrates
(acetophenone vs benzophenone) and electron-donating
substituents such as methoxy-groups resulted in higher
conversions (98%) after 18 h. Conversions were lower with
substrates containing electron-withdrawing groups such as 4′-
chloro- and 4′-nitro-acetophenone (entries 2 and 3). In
G
DOI: 10.1021/acs.organomet.9b00178
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 4), indicating a critical role of the tethered aNHC ligand
for imparting catalytic activity.
Table 3. Substrate Screening in the Transfer Hydrogenation
Reaction using 1 as Catalyst Precursor
a
In contrast to transfer hydrogenation catalysis, the alcohol
dehydrogenation activity of the ruthenium(II) center is
dependent neither on the nature of the ancillary ligand set
(p-cym/Cl vs Cp/PPh3) nor on the type of chelating group
(N-alkenyl vs N-picolyl). Conversions after 4 h were generally
60 5% (72 5% after 24 h) when using the abnormal NHC
Ru(II) complexes (Figure S38), reaching a modest TOFmax = 9
h−1. Again a notable decrease in catalytic activity is observed
with the complexes containing substantial amounts of the
normal NHC Ru(II) complex (4/5 and 7/8, entries 4 and 6).
When the essentially equal activity of all aNHC Ru complexes
b
conversion (%)
entry
substrate
benzophenone
acetophenone
4′-chloroacetophenone
4′-nitroacetophenone
4′-hydroxyacetophenone
2′-hydroxyacetophenone
4′-methylacetophenone
4′-methoxyacetophenone
2 h
18 h
1
2
3
4
5
6
7
8
63
56
34
18
24
25
42
47
96
98
87
53
71
76
96
98
1
is considered, and the H NMR spectroscopically determined
ratios of complexes 4/5 (2:1) are considered, the catalytic
activities of the normal NHC complex in the mixture of 4/5 is
estimated to be approximately half of that of the abnormal
NHC complex. The same accounts for the 1:1 mixture of 7/8,
with the normal NHC imparting only approximately half the
catalytic activity from that of the aNHC ligand.
a
General conditions: Ketone (0.6 mmol), iPrOH (5 mL), KOH (5
b
mol %), [1] (5 mol %), anisole (0.3 mmol), 110 °C. Determined by
GC, based on the average of at least two runs; calibrations indicate
that conversions are identical to yields under these conditions.
According to the classical transfer hydrogenation and
alcohol oxidation mechanisms for Ru(II) catalysts, a
monohydride intermediate is proposed.26 From the catalysis
results of this study it is concluded that generation of available
coordination sites for catalysis is mediated in different ways
depending on the complex: (i) For all p-cymene complexes,
dehalogenation by either KOH or KOtBu generates an
available site for substrate coordinationno NMR resonances
for free p-cymene could be observed for either transfer
hydrogenation or alcohol oxidation reactions; (ii) For all Cp
complexes, loss of PPh3 as OPPh3 (according to 31P NMR
spectra) generates an available site for catalysis; (iii) In both
transfer hydrogenation and alcohol oxidation reactions, no
hydride signals of either p-cymene or Cp catalysts could be
addition, the conversions of 2′- and 4′-hydroxyacetophenone
substrates were relatively low, presumably because the acidic
phenol functionality impedes substrate coordination and
instead forms an inactive Ru phenolate species.
Alcohol Oxidation. The series of complexes 1−10 was
also tested in the anaerobic oxidation of secondary alcohols,
where normal NHC analogues of complex 6 featured high
activity with TOFmax up to 330 h−1.2c,21 The catalytic reactions
were performed using o-dichlorobenzene as solvent and
hexamethylbenzene (0.1 mmol) as internal standard, at a
temperature of 150 °C. In general, all complexes catalyze the
oxidation at a 5 mol % loading and afforded moderate yields of
the corresponding ketone (Table 4). Similar to the transfer
hydrogenation reaction, the NHC-free complexes ([(p-cym)-
RuCl2]2 or [CpRuCl(PPh3)2]) showed very low catalytic
activities (≤8% after 18 h of reaction time; entries 9 and 10 in
1
observed in their H NMR spectra, and hence it is proposed
that the monohydride intermediate catalytic species are not the
resting-state intermediate species.
CONCLUSIONS
■
a
Variation of the arene ligand (p-cymene vs cyclopentadienyl)
and of the chelating tether of aNHC ligands (alkenyl vs
picolyl) provided access to six unique half-sandwich aNHC
Ru(II) complexes. In addition, Ag-mediated C(2)-demethyla-
tion resulted in the identification of two normally bound NHC
Ru(II) side products. Symmetrization of the N-alkene
substituents of the aNHC ligand, as well as employing an
iPr-group on the C(2)-position of the imidazolium precursor,
prevented C(2)-dealkylation and allowed for the selective
C(4)-ruthenation for both the p-cymene and cyclopentadienyl
Ru(II) precursors. Preliminary catalytic studies involving
transfer hydrogenation suggest a greater impact of vacant
coordination sites available via halide substitution (p-cymene
Ru(II) complexes) than via reversible alkene and/or
phosphine dissociation (cyclopentadienyl Ru(II) complexes).
Furthermore, a slight deactivating effect has been observed for
both p-cymene and cyclopentadienyl aNHC Ru(II) complexes
when employing the less labile N-picolyl tether compared to
alkenyl chelating groups and when introducing a methyl group
on the aNHC ligand C(5) position. The transfer hydro-
genation results indicate that chelating aNHC ligand systems
provide a dynamic platform for the development of active,
selective, and long-lived catalysts.
Table 4. Oxidation of 1-Phenylethanol using 1−10
b
conversion (%)
entry
complex
4 h
24 h
1
2
3
4
5
6
7
8
9
1
2
3
4/5
6
7/8
9
10
63
60
58
50
62
43
57
58
77
73
70
62
73
56
74
68
c
c
d
e
([(p-cym)RuCl2]2
[CpRuCl(PPh3)2]
nd
nd
8
e
d
10
8
a
General conditions: 1-Phenylethanol (1 mmol), o-dichlorobenzene
(5 mL), KOtBu (5 mol %), [Ru] (5 mol %), hexamethylbenzene (0.1
mmol), 150 °C. Determined by GC, based on the average of at least
two runs. Mixture of complexes as isolated after purification. nd =
b
c
d
e
not detected. After 18 h.
H
DOI: 10.1021/acs.organomet.9b00178
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(NCH2), 78.1 (CH2), 78.9 (CCH3), 95.0 (CcymH), 97.1
(CcymH), 101.7 (CcymH), 108.9 (CimiMe), 117.2 (Ccym−iPr), 133.4
(NCN), 168.8 (C−Ru). 31P{1H} NMR (CDCl3): δP = −144.3
EXPERIMENTAL SECTION
■
General. All experiments were performed under an argon
atmosphere using standard Schlenk techniques. Solvents were dried
and distilled from appropriate drying agents prior to use. The new
imidazolium chloride salts [H(L1−L4)]Cl were synthesized and
purified according to standard literature procedures (see Supporting
Information).27 The metal precursor [CpRuCl(PPh3)2] was synthe-
sized according to the reported method.22 All other chemicals were
purchased from commercial suppliers and used without further
1
(septet, JPF = 713 Hz, PF6). CHN found (calcd) for
[C20H30ClF6N2PRu] × 0.63CHCl3: C, 38.24 (37.84), H, 4.30
(4.71), N, 4.31 (4.28)%. HR-MS (ESI): m/z 435.1152 (2+) calcd
for C20H30ClN2Ru 435.1141.
1
Complex 3. Yield: 0.17 g, 23%. H NMR ((CD3)2CO): δH = 1.70
(d, 2JHH = 12 Hz, 1H, NCH2), 1.80 (s, 3H, CCH3), 2.31 (d, 3JHP
=
12 Hz, 1H, CH2), 2.34 (s, 3H, Cimi−CH3), 3.71 (s, 3H, NCH3),
3.80 (s, 1H, CH2), 3.88 (d, 2JHH = 12 Hz, 1H, NCH2), 4.97 (s, 5H,
C5H5), 7.02 (s, 1H, CimiH), 7.38−7.49 (m, 15H, HPh). 13C{1H} NMR
(CDCl3): δC = 10.2 (CCH3), 31.2 (Cimi−CH3), 45.7 (NCH3), 56.8
(NCH2), 76.3 (CH2), 76.6 (CCH3), 87.8 (C5H5), 128.0
(CimiH), 128.7 (CPh), 130.3 (CPh), 133.5 (CPh), 135.2 (ipso-CPh),
139.2 (NCN), 147.0 (C−Ru). 31P{1H} NMR (CDCl3): δP = −144.2
(septet, 1JPF = 708 Hz, PF6), 56.8 (s, PPh3). CHN (%) found (calcd)
for [C32H34F6N2P2Ru] × 2.22(CH3)2CO: C, 54.06 (54.46), H, 5.98
(5.59), N, 3.23 (3.29)%. HR-MS (ESI): m/z 579.1514 (3+) calcd for
C32H34N2PRu 579.1503.
1
purification. H (300/400 MHz) and 13C{H} (76/100 MHz) NMR
spectra were recorded on either a Bruker ARX-300 or a Ultrashield
Plus 400 AVANCE 3 spectrometer. All measurements were
performed at ambient temperature (298 K), unless otherwise noted.
Chemical shifts were referenced to SiMe4 by the internal residual
protio solvent resonances. Solid-state Fourier transform infrared (FT-
IR) experiments were performed on a PerkinElmer Spectrum RXI FT-
IR spectrometer as pressed KBr pellets in air. Microanalytical analyses
(%CHNS) were obtained using a Thermo Scientific Flash 2000
elemental analyzer fitted with a thermal conductivity detector (TCD).
Although some of these results are outside the range viewed as
establishing analytical purity (values within 0.4%), they are provided
to illustrate the best values obtained currently. GC analyses were
performed on a Shimadzu GC-2010 fitted with a flame ionization
detector (FID) using a TRB-780143 capillary column (30 m, 0.25
mm ID, thickness 0.25 μm) and an AOC-20i auto injector.
Electrospray mass spectra (ESI-MS) were recorded on a Micro-
massQuatro LC instrument.
Complexes 4 and 5. Complexes 4 and 5 eluted together in a 2:1
ratio. Combined yield: 0.28 g (39%); by NMR 24% (4), 15% (5).
1
Spectroscopic data of 4: H NMR (CDCl3): δH = 1.45 (d, 2JHH = 12
3
Hz, 1H, NCH2), 1.85 (s, 3H, CCH3), 2.03 (d, JHP = 12 Hz, 1H,
CH2), 2.25 (s, 3H, Cimi−CH3), 2.47 (s, 3H, NCH3), 2.92 (s, 3H,
Cimi−CH3), 3.53 (d, 2JHH = 12 Hz, 1H, NCH2), 3.78 (s, 1H, CH2),
5.02 (s, 5H, C5H5), 7.31−7.49 (m, 15H, HPh). 13C{1H} NMR
((CD3)2CO): δC = 9.3 (CCH3), 31.8 (Cimi−CH3), 33.9 (Cimi
−
General Synthesis of Half-Sandwich NHC Ru(II) Complexes
1−10. A suspension of the appropriate imidazolium chloride salt
[H(L1−L4)]Cl (1 mmol) in CH2Cl2 (20 mL) containing Ag2O (0.23
g, 1 mmol) was stirred at 30 °C for 12 h in the absence of light. The
mixture was filtered, to which either [(p-cym)RuCl2]2 (0.5 mmol) or
[CpRuCl(PPh3)2] (1 mmol) was added and heated under reflux for
either 5 h ([(p-cym)RuCl2]2) or 36 h ([CpRuCl(PPh3)2]). The crude
reaction mixture was filtered, and the filtrate was concentrated in
vacuo to 5 mL. Acetone (15 mL) and NH4PF6 (0.16 g, 1 mmol) were
added, and the reaction mixture was stirred for a further hour. The
mixture was evaporated in vacuo and purified by silica gel column
chromatography using gradient elution with CH2Cl2/acetone. Yellow
to dark yellow solids were obtained.
CH3), 47.6 (NCH3), 56.6 (NCH2), 78.4 (CH2), 79.8 (CCH3),
88.2 (C5H5), 117.8 (CimiMe), 128.7 (CPh), 130.5 (CPh), 131.7 (CPh),
133.5 (ipso-CPh), 134.3 (NCN), 149.7 (C−Ru). 31P{1H} NMR
1
(CDCl3): δP = −144.2 (septet, JPF = 708 Hz, PF6), 56.7 (s, PPh3).
1
Spectroscopic data of 5: H NMR (CDCl3): δH = 1.70 (s, 3H, 
CCH3), 2.19 (s, 3H, Cimi−CH3), 2.51 (s, 3H, NCH3), 4.36 (s, 1H, 
CH2), 4.49 (s, 1H, NCH2), 4.64 (s, 1H, CH2), 5.02 (s, 5H, C5H5),
2
5.03 (d, JHH = 12 Hz, 1H, NCH2), 6.74 (s, 1H, CimiH), 7.52−7.66
(m, 15H, HPh). 13C{1H} NMR ((CD3)2CO): δC = 9.5 (CCH3),
31.4 (Cimi−CH3), 47.6 (NCH3), 54.4 (NCH2), 75.8 (CH2), 81.4
(CCH3), 87.8 (C5H5), 114.6 (CimiMe), 121.0 (CimiH), 128.6
(CPh), 130.3 (CPh), 131.9 (CPh), 132.7 (ipso-CPh), 174.0 (C−Ru).
1
31P{1H} NMR (CDCl3): δP = −144.2 (septet, JPF = 708 Hz, PF6),
1
Complex 1. Yield: 0.28 g, 49%. H NMR ((CD3)2CO): δH = 1.32
57.7 (s, PPh3). CHN (%) found (calcd) for [C33H36F6N2P2Ru] ×
0.5[C32H34F6N2P2Ru] × 0.31CHCl3: C, 53.22 (52.82), H, 4.26
(4.79), N, 4.01 (3.76)%. HR-MS (ESI): m/z 593.1619 (4+), 579.1501
(5+) calcd for C33H36N2PRu (4) 593.1661, C32H34N2PRu (5)
579.1503.
3
3
(d, JHH = 6 Hz, 3H, CH(CH3)2), 1.34 (d, JHH = 6 Hz, 3H,
CH(CH3)2), 1.79 (s, 3H, CCH3), 1.98 (s, 3H, cym−CH3), 2.64 (s,
3
3H, Cimi−CH3), 2.78 (septet, JHH = 6 Hz, 1H, CHMe2), 3.68 (d,
2JHH = 9 Hz, 1H, NCH2), 3.85 (s, 3H, NCH3), 4.07 (s, 1H, CH2),
4.44 (d, 2JHH = 9 Hz, 1H, NCH2), 5.27 (d, 3JHH = 6 Hz, 1H, CcymH),
5.71 (s, 1H, CH2), 5.80 (d, 3JHH = 6 Hz, 1H, CcymH), 6.50 (d, 3JHH
1
Complex 6. Yield: 0.22 g, 38%. H NMR (CDCl3): δH = 1.20 (d,
3JHH = 9 Hz, 3H, CH(CH3)2), 1.22 (d, 3JHH = 9 Hz, 3H, CH(CH3)2),
3
1.85 (s, 3H, Cimi−CH3), 2.62 (s, 3H, cym−CH3), 2.75 (septet, 3JHH
=
= 6 Hz, 1H, CcymH), 6.65 (d, JHH = 6 Hz, 1H, CcymH), 7.41 (s, 1H,
CimiH). 13C{1H} NMR (CDCl3): δC = 14.1 (CCH3), 19.6 (cym−
CH3), 21.0 (CH(CH3)2), 22.2 (CH(CH3)2), 29.6 Cimi−CH3), 31.8
(cym CMe2), 54.1 (NCH3), 60.3 (NCH2), 80.2 (CH2), 81.1 (
9 Hz, 1H, CHMe2), 3.62 (s, 3H, NCH3), 4.88 (d, 2JHH = 15 Hz, 1H,
NCH2), 5.17 (d, 3JHH = 6 Hz, 1H, CcymH), 5.35 (d, 2JHH = 15 Hz, 1H,
NCH2), 5.52 (s, 2H, Ccy3mH), 5.57 (d, 3JHH = 6 Hz, 1H, CcymH), 6.58
3
CHMe), 96.6 (CcymH), 101.7 (CcymH), 115.2 (CimiH), 121.5 (Ccym
−
(s, 1H, CimiH), 7.32 (t, JHH = 6 Hz, 1H, C5H4N), 7.61 (d, JHH = 6
iPr), 122.7 (Ccym−Me), 137.6 (NCN), 171.1 (C−Ru). 31P{1H} NMR
Hz, 1H, C5H4N), 7.77 (t, 3JHH = 6 Hz, 1H, C5H4N), 9.36 (d, 3JHH = 6
Hz, 1H, C5H4N). 13C{1H} NMR (CDCl3): δC = 9.52 (Cimi−CH3),
18.1 (cym−CH3), 22.3 (CH(CH3)2), 22.5 (CH(CH3)2), 30.6 (cym
CMe2), 34.4 (NCH3), 53.0 (NCH2), 82.0 (CcymH), 86.1 (CcymH),
88.6 (CcymH), 99.5 (CcymH), 107.1 (Cpy), 124.6 (CimiH), 125.1
(Ccym−iPr), 125.2 (Cpy), 125.5 (Ccym−Me), 139.1 (NCN), 141.9
(Cpy), 150.0 (Cpy), 156.0 (Cpy), 157.8 (C−Ru). 31P{1H} NMR
(CDCl3): δP = −144.3 (septet, 1JPF = 709 Hz, PF6). CHN (%) found
(calcd) for [C21H27ClF6N3PRu] × 0.47CH2Cl2: C, 40.52 (40.12), H,
4.52 (4.38), N, 6.17 (6.54)%. HR-MS (ESI): m/z 458.0948 (6+)
calcd for C21H27ClN3Ru 458.0941.
1
(CDCl3): δP = −144.2 (septet, JPF = 713 Hz, PF6). CHN found
(calcd) for [C19H28ClF6N2PRu] × 1.75H2O × 0.5CH2Cl2: C, 36.26
(36.60), H, 5.25 (5.12), N, 3.98 (4.38)%. High-resolution mass
spectrometry (HR-MS) (ESI): m/z 421.0962 (1+) calcd for
C19H28ClN2Ru 421.0985.
1
Complex 2. Yield: 0.22 g, 38%. H NMR ((CD3)2CO): δH = 1.19
3
3
(d, JHH = 6 Hz, 3H, CH(CH3)2), 1.24 (d, JHH = 6 Hz, 3H,
CH(CH3)2), 2.18 (s, 3H, CCH3), 2.23 (s, 3H, cym−CH3), 2.29 (s,
3
3H, Cimi−CH3), 2.35 (s, 3H, Cimi−CH3), 2.79 (septet, JHH = 6 Hz,
1H, CHMe2), 3.31 (s, 1H, CH2), 3.87 (s, 3H, NCH3), 4.27 (d,
2
2JHH = 15 Hz, 1H, NCH2), 4.34 (d, JHH = 15 Hz, 1H, NCH2), 4.75
Complexes 7 and 8. Complexes 7 and 8 were obtained as a 1:1
mixture that coelutes by column chromatography and cocrystallizes
upon recrystallization. Combined yield: 0.28 g, 37%; by NMR: 19%
(7); 18% (8). Spectroscopic data of 7: 1H NMR (CDCl3): δH = 2.17
(s, 3H, Cimi−CH3), 3.35 (s, 3H, NCH3), 4.75 (s, 5H, C5H5), 4.80 (d,
(s, 1H, CH2), 5.64 (d, 3JHH = 6 Hz, 1H, CcymH), 5.82 (d, 3JHH = 6
Hz, 1H, CcymH), 6.55 (d, 3JHH = 6 Hz, 1H, CcymH), 6.64 (d, 3JHH = 6
Hz, 1H, CcymH). 13C{1H} NMR (CDCl3): δC = 10.1 (CCH3), 18.9
(cym−CH3), 22.0 (CH(CH3)2), 23.9 (CH(CH3)2), 27.8 (Cimi
−
2
CH3), 31.4 (cym CMe2), 34.9 (Cimi−CH3), 54.5 (NCH3), 70.2
2JHH = 18 Hz, 1H, NCH2), 5.38 (d, JHH = 18 Hz, 1H, NCH2), 6.64
I
DOI: 10.1021/acs.organomet.9b00178
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Organometallics
Article
(s, 1H, CimiH), 6.87−6.92 (m, 5H, HPh), 7.21−7.36 (m, 10H, HPh),
1H NMR spectroscopy for selected catalytic experiments. More
consistent values were determined using GC analysis, and conversions
were determined relative to anisole as the internal standard. All yields
are based on the average of at least two runs.
Typical Procedure for Alcohol Oxidation Catalysis. To a
round-bottom flask containing 1-phenylethanol (0.121 mg, 1
mmol), KOtBu (6 mg, 0.05 mmol, 5 mol %), hexamethylbenzene as
internal standard (18 mg, 0.1 mmol), and the ruthenium complex
(0.05 mmol, 5 mol %) was added o-dichlorobenzene (4 mL), and the
reaction mixture was stirred at 150 °C for the time indicated. Aliquots
(0.05 mL) of the reaction mixture were diluted with 2-propanol, and
conversions were determined by GC analysis relative to hexame-
thylbenzene as the internal standard. All yields are based on the
average of at least two runs.
X-ray Crystallography of [HL1]Cl, [HL4]Cl, 1, 3, 5, 6, 7/
8.CH2Cl2, 7/8.CHCl3 and 10. Single-crystal diffraction studies of the
compounds were done using either Quazar multilayer optics
monochromated Mo Kα radiation (k = 0.710 73 Å) on a Bruker
D8 Venture kappa geometry diffractometer with duo Iμs sources, a
Photon 100 CMOS detector, and APEX II control software28
([HL1]Cl, [HL4]Cl, 3, 6, 7/8·CH2Cl2, 7/8·CHCl3, and 10), or an
Oxford Diffraction SuperNova area-detector diffractometer29 using
mirror optics monochromated Mo Kα radiation (λ = 0.710 73 Å) and
Al filtered30 (1, 5). X-ray diffraction measurements were performed at
either 123(2) K (1, 5), 150(2) K ([HL1]Cl, 3, 6, 7/8·CH2Cl2, 7/8·
CHCl3), or 298(2) K ([HL4]Cl, 10). Data reduction was performed
using the CrysAlisPro29 program. The intensities were corrected for
Lorentz and polarization effects, and an absorption correction based
on the multiscan method using SCALE3 ABSPACK in CrysAlisPro29
was applied (1, 5). For the remaining compounds: Data reduction
was performed using SAINT+,28 and the intensities were corrected for
absorption using SADABS.28 The structures were solved by direct
methods using SHELXT31 using the SHELXL-2014/732 program. The
non-hydrogen atoms were refined anisotropically. All H atoms were
placed in geometrically idealized positions and constrained to ride on
their parent atoms. For a table containing the data collection and
refinement parameters, see Tables S1−S3 in Supporting Information.
Crystallographic data for all structures have been deposited with the
Cambridge Crystallographic Data Centre (CCDC) as supplementary
publication numbers) 1843098 ([HL1]Cl), 1843100 ([HL4]Cl),
1843096 (1), 1843103 (3), 1843097 (5), 1843099 (6), 1843101 (7/
8·CH2Cl2), 1843102 (7/8·CHCl3), and 1843104 (10).
3
7.41−7.54 (m, 3H, Hpy), 8.25 (d, JHH = 6 Hz, 1H, Hpy). 13C{1H}
NMR (CDCl3): δC = 9.6 (Cimi−CH3), 37.3 (NCH3), 55.1 (NCH2),
77.6 (C5H5), 122.5 (CimiH), 125.4 (C5H4N), 128.1 (CPh), 129.4
(CPh), 129.8 (CPh), 133.1 (Cpy), 133.5 (ipso-CPh), 135.3 (Cpy), 135.9
(NCN), 137.6 (Cpy), 157.3 (Cpy), 159.9 (C−Ru). 31P{1H} NMR
1
(CDCl3): δP = −144.2 (septet, JPF = 708 Hz, PF6), 56.8 (s, PPh3).
Spectroscopic data of 8: 1H NMR (CDCl3): δH = 2.99 (s, 1H,
NCH3), 4.40 (s, 5H, C5H5), 4.85 (d, 2JHH = 18 Hz, 1H, NCH2), 5.34
2
(d, JHH = 18 Hz, 1H, NCH2), 6.34 (s, 1H, CimiH), 6.44 (s, 1H,
CimiH), 6.87−6.92 (m, 5H, HPh), 7.21−7.36 (m, 10H, HPh), 7.41−
3
7.54 (m, 3H, Hpy), 7.69 (d, JHH = 6 Hz, 1H, Hpy). 13C{1H} NMR
(CDCl3): δC = 33.9 (NCH3), 53.9 (NCH2), 77.2 (C5H5), 123.3
(CimiH), 125.3 (CimiH), 128.0 (Cpy), 128.5 (CPh), 128.7 (CPh), 130.0
(CPh), 133.0 (Cpy), 133.6 (ipso-CPh), 135.3 (Cpy), 137.0 (Cpy), 156.7
(Cpy), 184.4 (C−Ru). 31P{1H} NMR (CDCl3): δP = −144.2 (septet,
1JPF = 708 Hz, PF6), 56.0 (s, PPh3). CHN (%) found (calcd) for
[C34H33F6N3P2Ru] × [C33H31F6N3P2Ru] × 2CH2Cl2: C, 49.03
(49.41), H, 4.47 (4.09), N, 5.23 (5.01)%. HR-MS (ESI): m/z
616.1520 (7+), 602.1279 (8+) calcd for C34H33N3PRu (7) 616.1460,
C33H31N3PRu (8) 602.1304.
1
Complex 9. Yield: 0.39 g, 62%. H NMR (CDCl3): δH = 1.23 (d,
3JHH = 6 Hz, 3H, CH(CH3)2), 1.25 (d, 3JHH = 6 Hz, 3H, CH(CH3)2),
3
3
1.29 (d, JHH = 6 Hz, 3H, CH(CH3)2), 1.34 (d, JHH = 6 Hz, 3H,
CH(CH3)2), 1.66 (s, 3H, CCH3), 1.74 (s, 3H, CCH3), 1.88 (s,
3
3H, cym−CH3), 2.69 (septet, JHH = 6 Hz, 1H, cym-CHMe2), 3.23
3
2
(septet, JHH = 6 Hz, 1H, imi−CHMe2), 3.73 (d, JHH = 12 Hz, 1H,
2
NCH2), 4.03 (s, 1H, CH2), 4.25 (d, JHH = 12 Hz, 1H, NCH2),
2
4.57 (s, 2H, CH2), 4.61 (d, JHH = 12 Hz, 1H, NCH2), 4.99 (d,
2JHH = 12 Hz, 1H, NCH2), 5.02 (d, 3JHH = 6 Hz, 1H, CcymH), 5.51 (s,
1H, CH2), 5.63 (d, 3JHH = 6 Hz, 1H, CcymH), 6.28 (d, 3JHH = 6 Hz,
3
1H, CcymH), 6.42 (d, JHH = 6 Hz, 1H, CcymH), 7.17 (s, 1H, CimiH).
13C{1H} NMR (CDCl3): δC = 15.2 (CCH3), 17.3 (CCH3), 18.8
(cym−CH3), 19.4 (CH(CH3)2), 19.6 (CH(CH3)2), 24.2 (CH-
(CH3)2), 24.9 (CH(CH3)2), 26.6 (CMe2), 30.7 (CMe2), 53.2
(NCH2), 56.1 (NCH2), 90.5 (CH2), 93.4 (CH2), 95.6 (
CCH3), 98.2 (CCH3), 99.4 (CcymH), 103.5 (CcymH), 113.4
(CcymH), 118.7 (CcymH), 125.7 (CimiH), 139.5 (NCN), 146.8
(Ccym−iPr), 148.0 (Ccym−Me), 152.3 (C−Ru). 31P{1H} NMR
1
(CDCl3): δP = −144.2 (septet, JPF = 710 Hz, PF6). CHN (%)
found (calcd) for [C24H36ClF6N2PRu] × 0.4CHCl3: C, 42.73
(42.98), H, 5.29 (5.38), N, 3.89 (4.11)%. HR-MS (ESI): m/z
489.1612 (9+) calcd for C24H36ClN2Ru 489.1614.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00178.
■
Complex 10. Yield: 0.43 g, 54%. 1H NMR (CDCl3): δH = 1.01 (d,
3JHH = 9 Hz, 3H, CH(CH3)2), 1.24 (d, 3JHH = 9 Hz, 3H, CH(CH3)2),
1.55 (s, 3H, CCH3), 1.74 (s, 3H, CCH3), 1.88 (d, 2JHH = 15 Hz,
S
1H, NCH2), 2.27 (d, 3JHP = 12 Hz, 1H, CH2), 3.11 (septet, 3JHH
=
9 Hz, 1H, CHMe2), 3.65 (s, 1H, CH2), 3.84 (d, 2JHH = 15 Hz, 1H,
2
NCH2), 4.47 (d, JHH = 15 Hz, 1H, NCH2), 4.48 (s, 1H, CH2),
Crystallographic details for all structures, crystallo-
graphic data files (in CIF format), CCDC identifier
codes (1843096−1843104), 1H, 13C, and 31P NMR
spectra, as well as time-resolved catalytic profiles (PDF)
2
4.58 (d, JHH = 15 Hz, 1H, NCH2), 4.79 (s, 5H, C5H5), 5.28 (s, 1H,
CH2), 6.75 (s, 1H, CimiH), 7.19−7.36 (m, 15H, HPh). 13C{1H}
NMR (CDCl3): δC = 19.4 (CCH3), 24.6 (CCH3), 25.6
(CH(CH3)2), 30.7 (CMe2), 45.1 (NCH2), 53.0 (NCH2), 53.4 (
CH2), 58.2 (CCH3), 68.0 (CH2), 77.2 (CCH3), 88.3 (C5H5),
113.0 (CimiH), 128.2 (CPh), 133.3 (CPh), 135.5 (CPh), 140.1 (ipso-
CPh), 146.3 (NCN), 148.6 (C−Ru). 31P{1H} NMR (CDCl3): δP =
−144.2 (septet, 1JPF = 710 Hz, PF6), 57.9 (s, PPh3). CHN (%) found
(calcd) for [C37H42F6N2P2Ru] × 0.5H2O: C, 55.35 (55.50), H, 5.62
(5.41), N, 3.13 (3.50)%. HR-MS (ESI): m/z 647.2150 (10+) calcd for
C37H42N2PRu 647.2130.
Accession Codes
CCDC 1843096−1843104 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Typical Procedure for Transfer Hydrogenation Catalysis. To
a round-bottom flask containing benzophenone (109 mg, 0.6 mmol),
base (4 mg, 0.03 mmol, 5 mol %), anisole as internal standard (22 μL,
0.2 mmol), and ruthenium complex (6 μmol, 1 mol %) was added 2-
propanol (4 mL), and the reaction mixture was heated to reflux for
the time indicated. Aliquots (0.05 mL) of the reaction mixture taken
at specified times were diluted with 2-propanol and analyzed by GC.
Conversions were determined relative to anisole as the internal
standard. Aliquots (0.05 mL) diluted with CDCl3 were analyzed by
AUTHOR INFORMATION
Corresponding Authors
*E-mail: martin.albrecht@dcb.unibe.ch. (M.A.)
*E-mail: marile.landman@up.ac.za. (M.L.)
■
ORCID
Martin Albrecht: 0000-0001-7403-2329
J
DOI: 10.1021/acs.organomet.9b00178
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Synthesis, Structural Elucidation, and Catalytic Activity. Organo-
metallics 2016, 35, 3421−3429.
́
Marile Landman: 0000-0003-4882-1142
Notes
(8) (a) Kruger, A.; Kluser, E.; Muller-Bunz, H.; Neels, A.; Albrecht,
̈ ̈
The authors declare no competing financial interest.
M. Chelating C4-Bound Imidazolylidene Complexes through
Oxidative Addition of Imidazolium Salts to Palladium(0). Eur. J.
ACKNOWLEDGMENTS
́
Inorg. Chem. 2012, 2012, 1394−1402. (b) Segarra, C.; Mas-Marza, J.
■
A.; Mata; Peris, E. Rhodium and Iridium Complexes with Chelating
C−C′-Imidazolylidene−Pyridylidene Ligands: Systematic Approach
to Normal, Abnormal, and Remote Coordination Modes. Organo-
metallics 2012, 31, 5169−5176.
This work has received support from the South African
National Research Foundation (M.L. Grant No. 93638, FPM
Grant No. 88594), the Central Research Fund of the Univ. of
Pretoria (M.L.), the Univ. of Pretoria Post Graduate Study
Abroad Programme (travel grant for F.M. to Univ Bern), and
the European Research Council (M.A., CoG 615653). Single-
crystal X-ray diffraction collections and further assistance by
Mr. D. Liles (Univ. of Pretoria), Dr. J. Hauser, and Dr. P.
Macchi (Univ. Bern) are gratefully acknowledged.
̈
(9) (a) Schaper, L.-A.; Ofele, K.; Kadyrov, R.; Bechlars, B.; Drees,
M.; Cokoja, M.; Herrmann, W. A.; Kuhn, F. E. N-Heterocyclic
̈
carbenes via abstraction of ammonia: ‘normal’ carbenes with
‘abnormal’ character. Chem. Commun. 2012, 48, 3857−3859.
(b) Kim, Y.; Lee, E. Activation of C−F bonds in fluoroarenes by
N-heterocyclic carbenes as an effective route to synthesize abnormal
NHC complexes. Chem. Commun. 2016, 52, 10922−10925.
(10) Martin, D.; Melaimi, M.; Soleilhavoup, M.; Bertrand, G. A Brief
Survey of Our Contribution to Stable Carbene Chemistry. Organo-
metallics 2011, 30, 5304−5313.
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